The present invention relates to a new nematic liquid crystal composition having positive dielectric anisotropy and more particularly, to a new nematic liquid crystal composition having positive dielectric anisotropy comprising a Schiff base-type nematic liquid crystal composition having negative dielectric anisotropy and a positive dielectric anisotropy-imparting agent.
Nematic liquid crystal materials (hereinafter referred to as N-liquid crystals) are used for electro-optical devices, because they are optically changed if an electric field is applied thereto. Electro-optical devices with use of N-liquid crystal may be divided into two groups according to dielectric anisotropy of the N-liquid crystal. One group comprises devices in which there is employed a dynamic scattering mode (DSM), i.e. an optical scattering phenomenon caused by collision of an ion and an Nn-liquid crystal molecule group when an electric field is applied to an N-liquid crystal having a molecular dipole moment in a direction substantially perpendicular to the longitude of the molecules, i.e. a nematic liquid crystal having negative dielectric anisotropy (hereinafter referred to as Nn-liquid crystal). Another group comprises devices in which there is employed an electric field effect mode (FEM), i.e. a change in lean or torsion of N-liquid crystal molecules caused by applying an electric field to an N-liquid crystal having a molecular dipole moment substantially parallel to the longitudinal direction of the molecules, i.e. a nematic liquid crystal having positive dielectric anisotropy (hereinafter referred to as Np-liquid crystal). Many of the latter FEM-type devices are so-called "torsion effect-type devices" (hereinafter referred to as TN-type device) wherein a change in torsion of Np-liquid crystal molecules (one of the electric field effect modes) is utilized. The Np-liquid crystal composition of the present invention is utilized for the TN-type device.
The TN-type device is prepared by applying a transparent conductive coating to one face of each of a pair of supports such as glass plates to obtain an electrode surface, then combining the pair of the plates in such a manner that the electrode surfaces are opposite to each other (distance between the electrode surfaces being usually 1 .mu. - 50 .mu.) to obtain a cell and thereafter introducing Np-liquid crystal to the cell to fill the cell with the liquid crystal. In such a case, the electrode surfaces have been treated previously so that they have a predetermined orientation according to oblique incidence deposition or rubbing methods. The electrode plates are combined together at a proper distance in such a manner that the orientations caused by the treatment of the electrode surfaces are substantially perpendicular to each other. In thus obtained TN-type device, Np-liquid crystal molecules are oriented in such a manner that longitudes of the molecules are parallel to the electrode surface and in the same direction as the orientation caused by the treatment of the electrode surface and, further, they have about 90.degree. torsion between the electrode surfaces. Pitch of the torsion of the Np-liquid crystal molecules is sufficiently larger as compared with wavelength of light and, accordingly, plan of polarization of linear polarized light perpendicular to the electrode plate is rotated by about 90.degree.C. while it passes through the TN-type device. Consequently, the TN-type device intercepts the light between two polarizers arranged in such a manner that the light-oscillation planes are parallel to each other and, on the other hand, it allows the light to pass between two polarizers arranged in such a manner that the light-oscillation planes are perpendicular to each other. If a voltage is applied to the TN-type device, the longitude of the Np-liquid crystal molecules is inclined to the electric field according to the voltage applied. At a voltage higher than a certain value, the Np-liquid crystal molecules are arranged so that the longitude thereof is substantially parallel to the direction of the electric field. Under such a condition, contrary to the case of no application of voltage, the TN-type device allows light to pass between the parallel polarizers but intercepts the light between the perpendicular polarizers. The TN-type device is thus changed from light-interception state to light-passing state or from the light-passing state to the light-interception state according to the application of voltage. This light modulation is utilized for displays.
The TN-type device is usually operated according to an alternating current electric field of a square wave or the like. This is an excellent method of preventing the Np-liquid crystal which is an organic compound from deterioration due to electrolysis thereof. A TN-type display apparatus comprising the TN-type device placed between the two polarizers and a driving circuit for the TN-type device can be used as the display for electronic computers, electronic desk computers, electronic watches and various other measuring instruments.
In most display apparatus for the above-mentioned various instruments, display devices such as fluorescent indicator tubes, cathode discharge tubes, etc. are utilized. N-liquid crystal display devices are advantageous over these conventional display devices in that they are operated at lower voltages. As display devices to be operated at a voltage equal to or lower than the voltage at which N-liquid crystal display devices are operated, there can be mentioned light emitting diodes, but the N-liquid crystal devices are excellent over light emitting diodes in respect to consumption of electric power. Especially, TN-type display devices are excellent over DSM type display devices in the point that they are operated at lower voltages and provide images having a higher stability. The fact that display device is operated at a lower voltage means that the following advantages can be attained in constructing an display system. Namely, the connection of the device with an IC circuit or the like can be facilitated, planing of circuits is easily accomplished, the circuit reliability is improved, and consumption of electric power is reduced. Accordingly, extensive research work has been directed toward development of TN-type devices which can be operated at lower voltages within a broad temperature range including room temperature and in its turn, toward development of Np-liquid crystal materials for TN-type devices that can be operated at such lower voltages and exhibit a liquid crystal state within a broad temperature range including room temperature.
Known Np-liquid crystal compositions used for said TN-type device are obtained usually be incorporating a positive dielectric anisotropy-imparting agent (hereinafter referred to as P-imparting agent) into an Nn-liquid crystal or Nn-liquid crystal composition. Those Np-liquid crystal compositions have been disclosed in, for example, Japanese Application Kokai No. 18783/1972 and Japanese application Kokai No. 38888/1974.